1. Field of the Invention
The present invention relates to a resistance change memory having an organic semiconductor layer.
2. Description of the Related Art
Recently, an MRAM (Magnetic Random Access Memory) is attracting attention as a new semiconductor memory. Since this MRAM can be formed only by a metal with which relatively large readout signals can be obtained, memory cells can be formed only by arranging MTJ (Magnetic Tunnel Junction) elements at the cross points of interconnections. Accordingly, the capacity can be increased by, e.g., stacking cells. An MRAM having this structure is called a cross-point type MRAM.
FIG. 47 is a schematic perspective view of a cross-point type MRAM according to the prior art. As shown in FIG. 47, a cross-point type memory cell is basically made up of a bit line 16, a word line 15, and an MTJ element 10 sandwiched between the bit line 16 and word line 15. The MTJ element 10 includes a free layer 11, a fixed layer 13, and a tunnel insulating layer 12 sandwiched between the free layer 11 and fixed layer 13.
Unfortunately, the above conventional cross-point type MRAM poses the following problem when a memory cell array is formed.
As shown in FIG. 48, to read out data written in an MTJ element 10a of a selected cell, switches SW(j) and SW(i) are turned on to select a bit line BL(j) and word line WL(i). As a result, a read current flows through the MTJ element 10a of the selected cell along the direction of the solid-line arrow.
In the cross-point type structure, however, the selected bit line BL(j) and word line WL(i) are connected to a plurality of MTJ elements, in addition to the MTJ element 10a of the selected cell. Therefore, a sneak current flows through MTJ elements 10b, 10c, and 10d along the directions of the dotted-line arrows.
In the cross-point type MRAM as described above, the ratio of the sneak current which does not flow the shortest distance is larger than the read current which flows through the actually selected cell, and this causes read errors. In addition, this problem becomes significant as the scale of the memory cell array increases.
As shown in FIG. 48, an electric current flows through the MTJ element 10a upward on the paper, whereas an electric current flows through the MTJ element 10c downward on the paper. An amorphous silicon diode can be used to prevent this sneak current which flows in the direction opposite to the MTJ element 10a of the selected cell. However, the present MRAM materials cause interface diffusion and deteriorate the characteristics by annealing at about 300° C. Accordingly, these materials cannot resist the deposition temperatures of amorphous silicon and polysilicon.